Rakuten is not alone in embracing open RAN architectures as a way to reduce cost and increase competitive agility in the Japanese 5G market. Market leader NTT Docomo has already run extensive trials based on the O-RAN Alliance’s interfaces and architecture, and has now announced the results of a collaborative effort to develop an O-RAN baseband unit.
This will be a central element of Docomo’s emerging virtualized RAN and its migration towards end-to-end cloud-native technology. Its new baseband unit (BBU) architecture can support centralized or distributed units (CUs or DUs) in the O-RAN architecture, and was developed with NEC and Samsung.
It can interface to an O-RAN compliant radio unit from any vendor, the MNO said, but also supports an open implementation of the 3GPP X2 interface, which means it can work with any existing 4G base station in its network.
The latter will be important to the vast majority of operators which are not greenfield like Rakuten Mobile, but need to have new virtualized and open RAN systems running alongside installed 4G or legacy base stations, potentially for some years, before the older equipment is fully depreciated or runs out of steam.
The semi-proprietary way in which X2, the standard interface between basebands, has been implemented by different vendors has been a major barrier to multivendor RANs, along with the similarly semi-proprietary CPRI interfaces between basebands and radio units in disaggregated networks. O-RAN Alliance, and other organizations such as Open Networking Foundation and Small Cell Forum, are developing new open fronthaul and base station interfaces which they believe will be deployed in a fully open way, because of the pressure from operators on their suppliers to do so.
While there are some brand new interfaces emerging, along with architectural elements like the new RAN Intelligent Controller (RIC), there is also a strong focus on opening up established specifications like X2, so that operators can continue to support existing 4G networks.
Docomo said of its latest innovation: “Expanding multivendor interoperability based on O-RAN open interface specifications will enable the most appropriate base stations to be used depending on deployment scenarios and taking advantage of specific vendor and equipment characteristics. This will drive the rapid and flexible development of 5G service areas.”
NEC is also a key partner of Rakuten Mobile – it is co-developing the operator’s 5G sub-6 GHz radio/antenna and its 5G core. But it has been a long-time central member of Docomo’s inner circle of suppliers, mainly Japanese, with which the operator has cooperated on cutting edge network developments since the 2G era.
Another trial saw NEC and Fujitsu – another close Docomo partner and now a radio supplier to the USA’s Dish – working together to demonstrate carrier aggregation across two vendors’ equipment using O-RAN. The vendors demonstrated carrier aggregation to a device that was served by radio units from different vendors – both RUs connected to the common baseband via O-RAN protocols.
The aggregation was achieved using spectrum in the 3.7 GHz and 4.5 GHz bands and reached peak speeds of 4.2Gbps. The results were presented during last month’s virtual O-RAN plugfest event, in which interoperability plugfests were held concurrently in Japan, China, India, Europe and North America.
Shingo Mizuno, vice head of system platforms at Fujitsu, commented: “This key milestone in the evolution of O-RAN will contribute greatly to the delivery of convenient 5G services and RAN openness”.
These breakthroughs fit into a long tradition of innovation by Japanese operators, working with their local suppliers on highly customized and advanced architectures. With Rakuten and the government touting Japanese leadership in the emerging open 5G ecosystem, however, this time the work done by NEC, Fujitsu and others may have more global impact.